AC_Fence: fixed 2 regressions from 4.5.x #41705

Job	Run time
build (base)	2m 30s
build (clang)	2m 20s
build-gcc-heli	3m 2s
autotest (sitltest-copter-tests1a)	7m 49s
autotest (sitltest-copter-tests1b)	9m 13s
autotest (sitltest-copter-tests1c)	11m 23s
autotest (sitltest-copter-tests1d)	9m 6s
autotest (sitltest-copter-tests1e)	8m 57s
autotest (sitltest-copter-tests2a)	8m 59s
autotest (sitltest-copter-tests2b)	29m 16s
autotest-heli (sitltest-heli)	10m 15s
	1h 42m 50s

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